Penninic Paleogeography from the Western toward the Eastern Alps-Still Open Questions?

摘要

The tectonic evolution of the Penninic units and the distal European margin during Late Cretaceous-Pal eogene time is very well constrained in the Western and Central Alps. In the Eastern Alps, the evolution of these units is controversial. I discuss three main problems: (1) the eastward continuation of the Northpenninic oceanic basin (Valais); (2) the eastward continuation of the Middlepenninic microplate (Brianconnais); and (3) the paleogeographic position of the Rbenodanubian flysch basin. (1) In the Eastern Alps, many palinspastic reconstructions display a Northpenninic position of the Glockner nappe within the Tauern window; the Matrei zone above is supposed to be Southpenninic. Because no continental units occur between the Glockner nappe and the Matrei zone, it is assumed that the Middlepenninic only extends to the area of the Engadme window. Other observations suggest that both the Glockner nappe and the Matrei zone are Southpenninic. (2) Older models suggested that the Zentralgneis in the central Tauern window with its sedimentary cover (Venediger nappe) forms the continuation of the Briangonnais. Recent tectonic reconstructions have shown that the Venediger nappe was part of the distal European margin. However, it was also suggested that certain gneiss lamellae above the Venediger nappe may have been derived from the Briangonnais. (3) The paleogeographic position of the Rhenodanubian flysch basin is commonly supposed to be Northpenninic. However, this was often based on the assumption that the Venediger nappe formed the continuation of the Briangonnais; the Rhenodanubian flysch was reconstructed in a position north of it. Based on micropaleontological and stratigraphic criteria, a Middlepenninic position along the northern margin of the Briangonnais was suggested by other workers. A third model proposed a Southpenninic position. At present, general reconstructions indicate that the Middlepenninic microplate ends east of the Engadine window. Thus, the North- and Southpenninic basins merge into a single basin east of this window. However, the occurrence of dismembered continental basement slices within the Tauern window has not vet been taken into account.